Both PET (Positron Emission Tomography) and MRI (Magnetic Resonance Imaging) are known techniques which can be used to image the body. In the case of PET, a solution is injected or taken into the body, the solution which contains molecules with positron emitting atoms. Therefore, once positron detector geometry is properly positioned around the patient, and once software is used to identify the location of the positron emission, the location and concentration of positron emitting molecules can be detected. One of the major molecules used is a derivative of glucose which can enter the cells as glucose but is not metabolised as glucose and so the regions of major activity in the body can be detected. If the molecules are specific for cancer then this imaging technique can be utilised to detect the location of the tumour. PET however does not provide any anatomical information, only information on the location of the positron emitting molecules. The PET images need to registered to the anatomy of the patient and this is where MRI can be very useful.
With MRI, a high field magnet, typically superconducting, is arranged in a torus configuration (like a donut) and with the patient lying down inside the magnet the magnetic field allows a pulsed and sequenced magnetic and EM field to probe the body to produce soft tissue images, which allow the trained radiologist to determine with high probability the anatomy of the patient. MRI is sometimes performed using contrast agents to provide even better contrast between different soft tissue types. MRI techniques are very good at detecting the anatomical location of many but not all tumours.
These two imaging techniques, MRI and PET, are orthogonal techniques, in that the PET detection technology can be constructed in such a way that it is unaffected by magnetic fields and the MRI system can be constructed to be unaffected by the PET techniques. For this reason, if an integrated PET and MRI detector system can be constructed, then the two systems can be operated in parallel or almost parallel manners in both space and time to allow for improved and more complete detection of tumour locations within the body.
PET and MRI can be used to obtain a functional image combined with anatomical image to provide the clinician with far more information than either technique alone. For example, positron emitting molecules can be developed which interact only with certain receptors in the brain and so these receptors can be observed by PET and then their actual location in the brain determined by the MRI.
PET imaging is also an established technique, with general knowledge being available as well as many patents on specific configurations of PET detectors.
U.S. Pat. No. 5,998,792 discusses a variable detector geometry in 1999, and describes PET imaging as follows:                In nuclear imaging, a radiopharmaceutical such as sup.99m Tc or .sup.201 T1 is introduced into the body of a patient. As the radiopharmaceutical decays, gamma rays are generated. These gamma rays are detected and used to construct a clinically useful image.        Positron emission tomography (PET) is a branch of nuclear medicine in which a positron-emitting radiopharmaceutical such as .sup.18 F-Fluorodeoxyglucose (FDG) is introduced into the body of a patient. Each emitted positron reacts with an electron in what is known as an annihilation event, thereby generating a pair of 511 keV gamma rays. The gamma rays are emitted in directions approximately 180 degrees apart, i.e. in opposite directions.        A pair of detectors registers the position and energy of the respective gamma rays, thereby providing information as to the position of the annihilation event and hence the positron source. Because the gamma rays travel in opposite directions, the positron annihilation is said to have occurred along a line of coincidence connecting the detected gamma rays. A number of such events are collected and used to reconstruct a clinically useful image.        Various detector systems have been used in PET imaging. One class of PET systems can be termed non-rotating systems. The most common non-rotating systems have one or more rings of detector elements disposed in a circle about the patient. Other non-rotating systems include cylindrical shell detector systems and hexagonal multi-plate systems. In each of these systems, the detector surrounds or nearly completely surrounds the object to be scanned. Since coincidence events at substantially all transverse angles within a slice can be detected, system sensitivity is does not vary much between locations in a transverse slice.        Another class of PET systems can be termed rotating systems. Partial ring systems and dual or triple head gamma camera systems with coincidence detection capabilities fall into this category. One type of partial ring system includes two arcs of radiation sensitive detectors disposed on a generally circular rotating gantry. The arcs of radiation detectors are fixed with respect to each other so that their centers are generally diametrically opposed, with a slight angular offset. Rotating systems have partial transverse angle coverage such that it is necessary to rotate the detectors about the patient (or vice versa) in order to sample the transverse angles needed to reconstruct fully tomographic images. The sensitivity of these systems thus varies across the detector field of view. This variation in sensitivity is taken into account during processing of the coincidence data.        
This description of the PET rotating and non-rotating geometries refers to the PET detector itself, and not to an integrated scanner using PET/MRI. U.S. Pat. No. 6,674,083 (Tanaka) issued Jan. 6, 2004 discusses a positron emission tomography apparatus.
The article “The New Challenges of Brain PET Imaging Technology”, written in Current Medical Imaging Reviews, 2006, 2, 3-13, authored by Habib Zaidi and Marie-Louise Montandon, demonstrates through a prototype construction that combined PET/MRI scanners are possible. “Whole-Body Imaging with PET/MRI”, European Journal of Medical Research, Jun. 30, 2004, page 309-312, states, regarding the combination of PET and MRI into a single scanner,                “The combination of these two excellent diagnostic imaging modalities into a single scanner offers several advantages in comparison to PET and MRI alone”.        
“Simultaneous PET and NMR”, The British Journal of Radiology, 75(2002),S53-S59 describes a small prototype PET scanner that is MR compatible is described. In this case, four meter sections of optical fiber are used to transport the scintillation signals to photomultiplier tubes that are removed from the high magnetic field area of the bore. They discuss the potential advantages for both temporal and spatial correlation. As they indicate in this paper,                “incorporation of PET and MR scanners into a single gantry would keep subject motion and tissue deformation between PET and MR acquisitions to an absolute minimum, as is the approach adopted for the combined PET and CT systems described elsewhere in this special issue (their references 2, 3)”        
Additional papers that include Simon Cherry as author are: “Simultaneous PET and MR imaging”, Y. Shao et al, Phys. Med. Biol. October 1997 42(10), 1965-70; “Contemporaneous positron emission tomography and MR imaging at 1.5T”, K. Farahani et al, J. Magn. Res. Imaging March 1999, 9(3):497-500; “A study of artefacts in simultaneous PET and MR imaging using a prototype MR compatible PET scanner”, RB Slates et al, Phys. Med. Biol. August 1999; 44(8):2015-27.
An additional publication is “MR-PET: Combining Function, Anatomy and More”, M. Schwaiger et al, September 2005 Medical Solutions, pp. 25-30. This publication provides a simple diagram which shows a Magnetom Espree with a modified bore liner assembly. In this bore liner assembly is shown an RF body coil, above which is the PET camera elements. The diagram indicates that PET is acquired with a ring inserted into the magnet—simultaneous acquisitions are possible.
In U.S. Pat. No. 4,939,464 (Hammer) issued Jul. 3, 1990 is disclosed a combination NMR/PET scanner which uses light pipes to communicate the scintillation events to the exterior of the magnet. The PET scanner ring is mounted in the magnet bore and is moveable relative thereto.
In U.S. Pat. No. 6,946,841 (Rubashov) issued Sep. 20, 2005 is disclosed a combination NMR/PET scanner for breast tissue which where the PET scanner ring is mounted outside the magnet bore and the patient is moved between the two scanning positions.
In U.S. Pat. No. 5,719,400 (Cherry) issued Feb. 17, 1998 is disclosed a high resolution detector for use in PET scanning which is suitable for use in combined PET/MRI systems.
Further, previous publications and patents have shown it is possible to move a high-field superconducting MRI system in translation.
In U.S. Pat. No. 5,735,278 (Hoult et al) issued Apr. 7, 1998 is disclosed a medical procedure where a magnet is movable relative to a patient and relative to other components of the system. The moving magnet system allows intra-operative MRI imaging to occur more easily in neurosurgery patients, and has additional applications for liver, breast, spine and cardiac surgery patients. The system is used as follows:
The magnet is at first some distance from the operating table, either in the side or back of the surgical room or perhaps within a holding bay area;
When imaging is required, the MRI magnet is advanced from its holding area and positioned in the imaging position over the table;
Images are taken and magnet is retracted to its holding area. Thus the MRI system consists of:
Magnet;
Rails, installed on the site;
Magnet mover system, which consists of a magnet carriage, cable carrier, and mover control system.